Tap for rolling and swaging internal screw threads



April 1, 1958 P. A. KOEHLER 2,828,493 I TAP FOR ROLLING AND SWAGING4 INTERNAL'SCREW THREADS Filed Oct. 50, 1956 W R..c 2 mm? 2 @0MM mk ,e a/a l. W b JM graf, .W

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United States Patent TAP FOR ROLLNG AND SWAGING INTERNAL SCREW THREADS Paul A. Koehler, Lyndhurst, Ohio Application October 30, 1956, Serial No. 619,178

9 Claims. (Cl. 10-140) This invention relates to taps for rolling internal screw threads and is an improvement on the invention shown in my co-pending patent application, Serial No. 547,732, filed November 18, 1955.

The type of tap for rolling internal threads with which this invention is concerned includes a thread forming roll having annular ribs for forming the internal thread. Since the ribs are annular and the finished thread is helical, the roll may contact the work piece at only one tangent point within the bore in which the threads are to be formed. This means that the thread forming roll must be smaller in diameter than the bore of the work piece. Also, the thread forming roll must be mounted upon a shaft upon which it may rotate. This shaft must necessarily be smaller in diameter than the thread forming roll but must be rigid enough to support the roll against inward deflection due to the tangential contact with the work piece.

From the foregoing it is apparent that there will be considerable sideward thrust transmitted from the work piece'to the roll and then to the shaft and resulting in a considerable force tending to bend the shaft inwardly. This is especially true when the forward end of the shaft is not supported, in which case there is a strong cantilever loading -upon the shaft.

Oue of the objects of the present invention is to provide a thread rolling tap in which there is a maximum of support for the thread forming roll against inward defiection.

It is another object to provide a tap of the type described in which the portion of the tap immediately behind the thread forming roll is piloted within and supported by the work piece in order to achieve maximum support for the thread forming roll.

It is another object of the invention to provide a tap for rolling internal threads wherein there is a thread forming roll having annular ribs and a support for the same which has helical, ribs for engaging the threads formed in the work piece for further supporting the thread forming roll.

It is another object of the invention to provide a tap of the type described in which there is a thread forming roll attached to a supporting member having helical threads for engaging the threads formed in the work piece and wherein there is a means for adjusting the pitch diameter relationship between the helical threads and the annular ribs.

Itis another object of the invention to provide a thread rolling tap comprising an annular ribbed portion for forming the threads and a spirally ribbed portion for automatically correcting any pitch error or thread form error which may be developed by the annular ribbed portion.

Other obiects of the invention will become apparent from the following description and from the drawings in which:

Figure 1 is a front view of one form of the invention;

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Figure 2 is a partial section view along lines 2-2 of Figure 3;

Figure 3 is an end view;

Figure 4 is an enlarged fragmentary view of the thread form;

Figure 5 is a partial section View of a modified form; and

Figure 6 is an end view of a portion of the modified I form.

In the form illustrated in Figures 1 through 3, the tap comprises a holder 11 having a cylindrical shank 12 terminating in an enlarged end 13 having helical threads 14 formed thereon. The helical threads 14 are preferably slightly rounded at their outer diameter, as shown in Figure 4. A plurality of flutes or grooves 15 traverse the threads 14.

The holder 11 also has a bore 16 therethrough which terminates at one end at a counterbore 17. As shown in Figure 2, the centerline or axis of the bore 16 and counterbore 17 is radially offset from the axis of the holder 1l and in one plane is parallel with the axis of the holder 11, while in a plane which is at right angles to the first mentioned plane, the axis of the bore and counterbore is at an angle 18 with respect to the axis of the holder. The angle 18 is equal to the helix angle of the thread 14.

Mounted Within the bore 16 and counterbore 17 is a shaft 19. A portion 20 of the shaft projects beyond the threaded end 13 of the holder. Mounted upon this projection is a needle bearing 21 and a thread forming roll 22. The latter has a shouldered recess 23 to receive a headed screw 24 which'is threadedly attached to the shaft 19 by means of the threads 25. The distance from the bottom wall 26 of the recess 23 to the inner face 27 of the roll 21 is slightly less than the distance from the head 28 of the screw 24 to the outer face 30 of the holder end 13, so that the thread roll 22 will be rotatable with respect to the holder 11 upon the needle bearing 20.

The thread forming roll 22 is formed with a plurality of annular thread forming ribs 31, 32, 33, and 34. In the present embodiment of the invention, four of these annular ribs are shown but obviously the number may be varied. Also, these ribs are shown of progressively larger diameter, with the largest, rib 34 being of a size and cross section shape to form and size the thread in the work piece 35 to substantially their final proportions. The ribs 33, 32, and 31 are preferably of diminishing outer diameter so that rib 31 will initiate the thread forming operation in the work piece and the other annular ribs will progressively enlarge the threads being formed and size them to substantially their final proportions.

The end face 30 of the holder 11 is formed perpendicular to the axis of the bore 16 and counterbore 17 and will thus be perpendicular to the axis of the holder when viewed as in Figure 2 and at an an angle 18 with a horizontal plane when viewed as in Figure l.

The thread forming roll 22 is so mounted upon the holder that a point 38 on the pitch diameter of rib 34 will be in horizontal alignment with a point 39 on the pitch diameter of the helical threads 14, as shown in Figure 4. Also, the crest of the rib 34 and the adjacent helical thread 14 at the points 38 and 39, as shown in Figure 2, are axially spaced a distance equal to the pitch of the helical thread 14.

The shaft 19 is retained within the holder 11 by means of la set screw 36 which engages -a groove 37 in the sha t.

ln operation the holder 11 and the work piece 35 are mounted in a machine for relative rotation with respect to each other and with their axes co-incident. As the holder 11 is advanced toward the work piece 35, the annular ribs 31, 32, 33, and 34 progressively engage the bore in the work piece and on one side thereof. Because the roll 22 is at an angle 18 with respect to the axis of the holder and henceof the work piece, the ribs 31-34 although annular, will form a helical thread in the work piece. This helical thread will be of the same diameter and pitch as the thread 14 and the latter will enter the work piece and have snug fit therewith. Because of the latter, the holder 11 will thus be piloted within the work piece and be supported thereby. Also, the helical ribs 14 will tend to iron to proper shape the threads being formed and correct any minor lead or pitch error therein. Further, upon becoming engaged with the threads being formed, the helical ribs 14 will actas a lead screw to accurately control further forward movement of the tap and thus automatically control the lead or'pitch developed by the annular ribs.

Upon initial entryof the thread forming roll 22 into the work piece, the tangential contact with the bore of the work piece will impose a heavy side load upon the roll 22 which will be transmitted through the bearing 20 to the enlarged portion 19 of the shaft. The latter in turn is supported by the enlarged head 13. Since the thread forming roll 22 is relatively short and the relatively large shaft end 19 is directly supported by the enlarged end 13, there will be adequate support to keep the thread forming roll 22 from being deflected inwardly, which would otherwise cause the thread in the work piece to be undersized. When the helical threads 14 engage the threads being formed, they contact the latter about the entire periphery and thus help to steady both the tap and work piece. i

ln the modification shown `in'Figures 5 and 6, a differ ent shaft 40 is used in place of shaft 16. This shaft has an enlarged portion 41 snugly fitting within the bore 17 of the enlarged end 13 of the holder, and it` also has a projecting portion 42 which is somewhat eccentric to the 2. A tap in accordance with claim l in which said helical thread is interrupted by an axially extending flute.

3. A tap in accordance with claim 1 in which said helical thread is rounded at its major diameter.

4. A tap in accordance with claim 1 in which there is a means for adjusting the horizontal alignment of said points.

5. A tap for roll forming internal threads comprising a holder having a helical thread, a thread forming roll rotatably mounted on said holder, said roll having a plurality of annular thread forming ribs of Smaller pitch diameter than the pitch diameter of said helical thread, said annularrribs being of increasing outer diameter inwardly from the outer end of said roll, said roll being so mounted that the annular rib of largest diameter has a point on its pitch diameter which is in substantial horizontal alignment with a point on the pitch diameter of said helical thread and with said annular rib of largest diameter axially spaced from said helical thread a distance at said points equal to the pitch of the helical thread. f

6. A tap for rolling internal threads comprising a holder having at one end thereof a transverse face and a helical thread adjacent thereto, a longitudinal bore in said portion 41 as shown in both Figures 5 andi6, the latter r figure being an end view of the shaft only.

The thread forming roll 22 is mounted Aupon the eccentric portion 42 in the same manner as described for the form shown in Figure 2. Because the shaft is mounted eccentrically with respect to the holder 11, and the portion 42 is eccentric with respect to the shaft 40, rotation of the shaft will cause the pitch line point 38 of the roll 22 to be adjusted radially inwardly or outwardly from the axis of the holder and hence the thread formed in the work piece can be controlled to be either slightly smaller, the same as, or slightly greater in pitch diameter` than the pitch diameter of the thread 14 on the holder. It is understood, of course, that to rotate the shaft 40, the set screw 36 is loosened and retightened when the shaft has been rotated to the desired position.

Although two preferred embodiments of the invention have been shown and described, it is apparent that various other changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

I claim:

1. A tap for rolling internal threads comprising a holder having a helical thread, a thread forming roll rotatably mounted on said holder, said roll having an annular thread forming rib of smaller pitch diameter than said helical thread, said roll being mounted with a point on the pitch diameter of said rib in substantial horizontal alignment with a point on the pitch diameter of said helical thread and with said thread forming rib axially spaced from said helical thread a distance at said points equal to the pitch of the helical thread.

holder and intersecting said transverse face, the longitudinal axis of said bore being offset from the longitudinal axis of the helical thread and parallel thereto in one plane and at an angle thereto in another plane, a shaft mounted in said bore and having a portion projecting therefrom,

a thread forming roll rotatably mounted on said shaft, said roll having an annular thread-forming rib, said roll being so mountedl that a point on thepitch diameter of said rib is inv substantial horizontal alignment with a point on the pitch diameter lof said helical thread and with said thread forming rib axially spaced from said helical thread a distance at said points equal to the pitch of the helical thread.

7. A tap in accordance with'claim 6 in which said projecting shaft portion is eccentric with said bore whereby rotation of said shaft within said bore will result in an adjustment of the horizontal alignment of said pitch diameter points.

8. A tap in accordance with claim 7 in which a bearing is interposed between the shaft and the roll to facilitate rotation of the latter.` n

9. A tap for rolling'internal threads comprising a hold- 4er having a helical thread, a threadrforming roll rotatably mounted on said holder, said roll having an annular thread forming rib of smaller pitch diameter than said helical thread, said roll being mounted with a point on the pitch diameter of said rib in substantial horizontal alignment with a point on the pitch diameter of said helical thread, said holder having an end face adjacent the helical thread, said endface being perpendicular to the axis of the holder in one plane and at an angle to said axis in another plane, and said roll having an end face in contact with said holder'end face, said roll end face being in a plane parallel to a plane containing the annular rib, and said point on the pitch diameter of the rib being axially spaced from said other point a distance equal to the pitch of the helical thread.

ReferencesCted in the file of this patent Y UNITED STATES PATENTS 

